Photos: Watch the ‘Shadow Dagger’ Solar Calendar Mark the Equinox

It took nearly 85 years for archaeologists to figure it out, but an inscribed cliff face in Arizona‘s Wupatki National Monument turns out to be a kind of 800-year-old timepiece, whose only moving parts are the orbit of the Earth and the wheeling of the sun through the sky.

First recorded in 1931 by anthropologist Harold Colton, the petroglyphs found along a landform known as Horseshoe Mesa remained poorly understood for much of the 20th century.

“The original 1931 records that Harold Colton created consisted of a 3-by-5 index card with one or two sentences ‘describing’ the site,” said David Purcell, a supervisory archaeologist at the Museum of Northern Arizona who’s leading a new re-investigation of the site.

After the 1930s, Horseshoe Mesa went largely unstudied until it was surveyed in the early 1980s, and researched again in the ’90s, but shortages of time and money conspired to keep the meaning of its petroglyphs a secret, at least for a time.

Then in 2015, after extensive study, collaboration with experts in prehistoric rock art, and uncounted hours documenting the petroglyphs with time-lapse photography and video, researchers say they’ve confirmed that the sandstone face known as Panel 50 is “definitely an observatory for the winter solstice and equinoxes.”

“Prehistoric observatories can measure important astronomical events through sighting or imaging,” Purcell explained.

“Imaging is the observation of sun or moon rays or shadows projected on a surface,” Purcell added.

“Panel 50 is an imaging calendar.”

Because of the natural happenstances of its orientation and its features, Panel 50 creates a unique interplay of light and shadow around the beginning of spring, fall, and winter.

“People living at Horseshoe Mesa would have observed these patterns and realized that they could be used to identify important solar milestones like the equinoxes, with the addition of the petroglyph elements,” Purcell said.

A natural outcropping of rock above the panel forms what researchers have dubbed a “shadow dagger” that bisects a spiral carved onto the cliff wall, while another shadow interacts with a set of eight circles pecked into the panel’s left side.

The sunwatchers who made these modifications were members of the Kayenta tradition, a group of Ancestral Puebloans who occupied northern Arizona from around 500 to 1300 CE, and lived at Horseshoe Mesa from the mid-12th to the beginning of the 14th century.

“Several solar calendars have been documented in the Verde Valley, which suggests that the knowledge of solar observation was greater than just the Chaco region, extending into the Sinagua and Hohokam areas,” he said.

The photos below show the observations made at Panel 50 on the spring equinox of 2015, as well as some of the artifacts found at Horseshoe Mesa.

Click any image to enlarge.

An Elko Corner-notched dart point — a style that dates from 1,500 to 4,000 years ago — is a sign of a Late Archaic presence at Horseshoe Mesa, archaeologists say. (Photo: D. Purcell/NPS. May not be used without permission.)

A petroglyph of a desert bighorn sheep found at Horseshoe Mesa is rendered in the Glen Canyon Linear style, another sign of Late Archaic culture, which dates back as much as 4,000 years (Photo: D. Purcell/NPS. May not be used without permission.)

On the equinox, sunlight does not reach Panel 50 until exactly 12:00 local time. (Photo: D. Purcell/NPS. May not be used without permission.)

Shortly after noon on the equinox, a “shadow dagger” starts to take shape over the spiral to the right, while another body of shadow approaches the group of circles to the left. (Photo: D. Purcell/NPS. May not be used without permission.)

As the “shadow dagger” narrows on top of the spiral, the shadow at left continues to move toward the circles. (Photo: D. Purcell/NPS. May not be used without permission.)

The left edge of the dagger bisects the spiral at the same time that the shadow at left aligns with the group of circles. (Photo: D. Purcell/NPS. May not be used without permission.)

During the afternoon, the shadow moves across the disks, dividing different numbers of them in shadow and others in light. Archaeologists think this marks a kind of “countdown” either to the equinox or an important time around it, such as the beginning of planting season. (Photo: D. Purcell/NPS. May not be used without permission.)

The day after the equinox, a bar of light appears to isolate the circles, before moving right to graze the spiral’s edge. By contrast, on the day before the equinox, this light bar does not touch the panel at all. (Photo: D. Purcell/NPS. May not be used without permission.)

Discussion

Good question, Steve. All of the features are naturally occurring. Purcell told me: “The various sunlight and shadow patterns that play out across the cliff face are natural; we found no evidence that the cliff projections that create the different shadows have been modified in any way.”

The roof leading to and over the disks appears to have been altered to allow the sun to hit the back wall. The roof is almost perpendicular to the wall and the almost right angle to the left side (facing) the disks also appears to have been man made. I am curious as to how it was determined that those angles were not man made. It seems odd that the sun will only enter to illuminate the disks on the one day of each equinox and that the effect and the features are completely “naturally occurring” ! What are the odds of finding such an occurrence in nature?

Most people don’t realize that determining the spring equinox is critical for planting crops. Calendars only work if there is a leap year and someone consistently marks off the days. Solar calendars are much more practical and much easier to design.

Correct me if I’m wrong, but a solar calendar should be easy to make. Place a stick in a smooth surface, like a rock. Local noon is the highest the sun is during a day. Mark the local noon of each day. The extremes of marks are the solstices. Using a string it is possible to mark the exact middle, giving equinoxes. Due to precession the actual spring equinox fluctuates between 19 to 21 March. When modern Gregorian calendar used. Precession causes the exact position of local noon on equinox and solstices to vary slightly. Exact time changes, it does not coincide exactly with local noon. But it should be good enough for planting.

Lee Pingel – Respectfully, I take that the stick is a gnomon casting a shadow that you are marking at “high noon”: How do you know when the sun is at it’s highest point in the sky? Now the sun is going up, now the sun is going down? In the middle is noon? Perhaps the shadow that is cast helps to tell when it is high noon? I think that high noon would be difficult to accurately sight, by eye. Then, how do you find a naturally occurring sun dagger that just happens to produce a sun dagger on the equinox that you can depict a spiral on that spot and the dagger will illuminate the spiral on the equinoxes? The sun moves fastest around the equinoxes and slowest around the solstices. This naturally occurring effect is only produced exactly on the equinoxes. How is it possible that the ancients recognized not just when the equinox was exactly and also how to use a naturally occurring sun dagger to mark those two days of the year and only those two days?